Transport phenomena concentration profile

Concentration profiles are very useful for understanding electrochemical phenomena. Yet It should be kept In mind that they make up only one part of those phenomena, since only diffusion Is visualized. For example, in the case described in appendix A.4.1, the profile of the NO3" anion does not appear flat, although this Ion Is not consumed at the electrodes. Indeed, even if diffusion has a tendency to carry this species from anode to cathode, the migration compensates for this phenomenon in exact terms. Finally, at steady state, there is no overall mass transport for these Ions. Another difficulty arises from the different interpretations of these concentration profiles, in terms of mass consumption or production in transient or steady states. 

For transporters, relatively low protein expression level and limited transport capacity makes for nonlinear, enzyme-like transport kinetics that is, the transport rate saturates with increasing substrate concentration. This phenomenon is the basis for the competitive interactions generally found for chemicals that are handled by one or more common transporters this is usually manifest as inhibition of the transport of one chemical by a structural analog. The extent to which these competitive interactions are important depends on the concentrations of the chemicals involved, their relative affinities for the common transporter, and their phar-macological/toxicological profiles (effects, effective concentrations, therapeutic index). Competition for transport is discussed below in the context of drug-drug interactions. 

The anomalous transport of solutes can be an important phenomenon to consider in estimations of the fate of agricultural chemicals. As the dispersivity, used in the traditional ADE, grows with depth, the spreading rate and the tailing of the solute concentrations can be larger than that estimated from short range observations. There may be a potential for chemicals to travel into and within the vadose zone faster Ilian expected from their movement in upper part of soil profile. Anom-... 

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